Trusted vs untrusted ssh/X connections

[Andrew DeFaria]

On 6/19/2014 7:37 PM, Larry Hall (Cygwin) wrote:
> On 06/19/2014 04:25 PM, Andrew DeFaria wrote:
>> This is something that's been bothering me for a long time and I
>> thought I
>> might look into it a little deeper. I'm not sure if I should post this
>> here
>> because it involves Cygwin/X but it also involves OpenSSh.
>
> Actually, this is probably off-topic since I don't see anything Cygwin-
> specific about setting up ssh/X connections.

But I get the "untrusted X11 forwarding" error only when I ssh from 
Cygwin -> Linux using -X.

>
>> When I ssh into a Linux machine using ForwardX11 I get those familiar
>> messages:
>>
>> Warning: untrusted X11 forwarding setup failed: xauth key data not
>> generated
>>
>> and according to
>> https://cygwin.com/ml/cygwin-xfree/2008-11/msg00154.html:
>> The warning can be silenced by using ssh -Y, since that
>> is what ssh -X is doing now anyway.
>>
>> However, I find -Y to be 20 times slower to log in than -X:
>
> This is probably a configuraton issue since when I ssh into my Linux
> system,
> login time is roughly equivalent.

Any ideas of what configuration file I should be looking and what that 
configuration option that would be?

>
>> Adefaria-lt:time ssh cm-job-ldev01 echo 'hi'
>> Warning: untrusted X11 forwarding setup failed: xauth key data not
>> generated
>> Warning: No xauth data; using fake authentication data for X11
>> forwarding.
>> /usr/bin/xauth:  error in locking authority file
>> /home/adefaria/.Xauthority
>> hi
>>
>> real    0m2.387s
>> user    0m0.075s
>> sys     0m0.446s
>> Adefaria-lt:time ssh -Y cm-job-ldev01 echo 'hi'
>> Warning: No xauth data; using fake authentication data for X11
>> forwarding.
>> hi
>> /usr/bin/xauth:  error in locking authority file
>> /home/adefaria/.Xauthority
>>
>> real    0m22.476s
>> user    0m0.091s
>> sys     0m0.477s
>> Adefaria-lt:
>>
>> Bonus points if you can help me get right of the other errors!
>
> I believe the error regarding the .Xauthority file has something to do with
> the permissions on the file.  As for the warning, I believe you want to
> unset DISPLAY on your PC, set X11Forwarding to "yes" on your Linux machine
> in your sshd_config file, and X11Forward to "yes" in you ssh_config file
> (for instance) on your PC.  At least, that's what I gathered from searching
> around on the net for the information. :-)

My experience with this is that if DISPLAY is not set and you ssh -X (or 
-Y) then on the other side DISPLAY is not set:

Adefaria-lt:echo $DISPLAY
:0
Adefaria-lt:ssh cm-job-ldev01 'echo $DISPLAY'
Warning: untrusted X11 forwarding setup failed: xauth key data not generated
Warning: No xauth data; using fake authentication data for X11 forwarding.
/usr/bin/xauth:  error in locking authority file /home/adefaria/.Xauthority
localhost:11.0
Adefaria-lt:unset DISPLAY
Adefaria-lt:ssh cm-job-ldev01 'echo $DISPLAY'

Adefaria-lt:

> I think it goes without saying that enabling X11Forwarding opens up
> some security holes in X.  Oops, looks like I said it anyway. ;-)

Inside the intranet, this is not a concern for me.
-- 
Andrew DeFaria
http://defaria.com


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